1. Field of the Invention
This invention relates to turbine machines and particularly to the flow losses imposed upon the working medium gases within the turbine exhaust case of such a machine.
2. Description of the Prior Art
A gas turbine engine is typical of turbine machines having an exhaust case through which working medium gases are flowed during operation of the engine. The exhaust case of such a machine conventionally comprises an inner cylindrical member forming the inner wall of the working medium flow path and an outer cylindrical member forming the outer wall of the flow path. A plurality of radially extending struts spans the flow path between the inner and outer cylinders to concentrically support the inner member.
Working medium gases discharging from the turbine into the exhaust case during operation of the machine have a residual velocity component in the tangential direction with respect to the inner wall of the flow path. The tangential velocity component of the medium gases detracts from the momentum increase producing a forward axial thrust and, in a gas turbine engine, is undesirable.
Conversion of the tangential velocity to axial velocity increases the axial thrust produced and is essential for optimum operation of the engine. In many constructions the tangentially flowing gases are redirected axially by the struts of the exhaust case. The struts are contoured to effect aerodynamically efficient turning of the flow. Resultantly, the flow path area between each pair of adjacent struts, as presented to the approaching medium, is less than the downstream discharge area between the struts and diffusion of the medium gases occurs during the turning process. During diffusion the velocity of the medium gases is slowed and the thickness of the boundary layer adjacent the radial walls increases.
The boundary layer is characterized by low velocity flows which vary in direction with respect to the mainstream velocity according to local pressure gradients within the boundary layer. The local pressure of the gases at any location within the boundary layer is that pressure which is imposed radially upon the boundary layer by pressure forces within the mainstream. In a region adjacent the pressure side surface of a case strut, the pressure forces within the mainstream are increased to include ram effects and that pressure is imposed radially upon the gases within the boundary layer. A pressure gradient between the pressure side surface of one strut and the opposing suction side surface of the adjacent strut is established.
The established circumferential pressure gradient encourages secondary flow of medium gases circumferentially about the inner wall through the boundary layer. One skilled in the art will recognize that the amount of secondary flow increases as the thickness of the boundary layer increases. Where a thick boundary layer is formed significant secondary flow occurs. The secondary flow runs askew to the mainstream flow and interfaces with the mainstream flow creating a zone of substantial turbulence. Aerodynamic mixing losses in the zone of turbulence are imposed upon the working medium gases and reduced overall engine performance results.
Continuing efforts are underway to decrease the mixing losses imposed upon the medium gases by reducing the amount of secondary flow within the exhaust case.